1. Technical Field
A system concerns an RF magnetic field (B1 magnetic field) generated by radio frequency (RF) pulses and reducing errors due to spin lattice relaxation and accelerating acquisition of a B1 map.
2. Description of the Related Art
A known magnetic resonance imaging device provides non-invasiveness, superior contrast imaged tissues compared to a computer tomography (CT) device, and does not form artifacts due to bone tissues. Furthermore, a magnetic resonance imaging (MRI) system is capable of capturing various tomography images in desired directions without need to relocate a target object. A known MRI device generates a magnetic resonance (MR) image using differences between characteristics of tissues of a target object and reflects differences between magnetic resonance characteristics of tissues facilitating recognition of tissues in an MR image.
Uniformity of a magnetic field (B1) generated by radio frequency (RF) pulses in an MRI device affects uniformity of an MR image. Therefore, B1 shimming is employed to improve B1 uniformity by driving respective coil devices included in a transmission RF coil using driving signals having different magnitudes and phases. Further, an RF coil system employs multiple circuit structures and includes a plurality of coil devices. It is necessary to determine spatial B1 magnetic field distribution to perform the B1 shimming referred to as B1 mapping. There are different known B1 mapping methods for determining whether a magnetic field (B1 magnetic field) formed by RF pulses is homogeneous or inhomogeneous or for determining uniformity. B1 mapping is an auxiliary imaging operation for acquiring information necessary for compensating RF pulses.
The different known B1 mapping methods include a double angle method (DAM) that acquires a B1 map by acquiring two MR images by using RF pulses having different magnitudes and by calculating a ratio between magnitudes of signals of the two MR images. In this case, to minimize spin lattice relaxation time (T1) dependency of MR image signals, a long repetition time (TR), which is about five times longer than T1, is required. Therefore, the DAM requires a long period of time for B1 mapping. Furthermore, spatial distribution of B1 magnetic fields is changed when a target object of an MRI operation is changed. Therefore, it is necessary to perform B1 mapping for imaging for each new target object, and thus it is necessary to perform B1 mapping quickly.
However, in order to perform B1 mapping quickly, it is necessary to reduce the TR of a RF pulse sequence. However, when the TR is reduced, magnitudes of MR image signals are also affected by T1 and T1 varies according to type of human body tissues. Therefore, efficiency of B1 shimming using a B1 map is reduced if errors due to spin lattice relaxation are not compensated for during B1 mapping.